Failure warning systems for navigational apparatus



June 9, 1959 A. P. GLENNY ET'AL FAILURE WARNING SYSTEMS FOR NAVIGATIONAL APPARATUS AFiled May 12. 1952 3 Sheets-Sheet 1 June 9, 1959- A. P, GLENNY ETAL 2,890,442

FAILURE WARNING SYSTEMS FOR NAVIGAT'IONAL APPARATUS Filed May 12, 1952 5 Sheets-Sheet 2 TOR E.

June 9 1959 v A, P, GLENNY ETAL 2,890,442

FAILURE WARNING SYSTEMS FOR NAVIGATIONAL APPARATUS Filed May 12. 1952 :s sheets-sheet s A Fg. I

United States Patent WARNING SYSTEMS FOR NAVIGATIONAL APPARATUS..

IArthurb Philip Glenny, Hanworth, and Frank Dove; St. 'Albans,-England, assignors, by--mesne assignmentatoY ThexSperry Gyroscope.Company, Limited,l Brentford,.-` England, la lcompany. of zGi'e-at'- Britain:

Application. May 121,. 1952, Serial No. 287,348'

Claims priority; 'application` Great Britailr May 11,11951 2 IClaims. (Cl. '34th-253) Thepresent invention relates to apparatus-'for use' in-the: navigation of mobile crafftof the 'kindI inwhichfa con-l trolr quantity or signalV representing information-`useful-zin` the Anavigation of'thev'craft" is derived, andftransmitted through asignal channelrto utilisation means whereiit isusedto exercisea control action.- Such'apparatus falls in lgeneral intoftwoclasses: automatic control-apparatus and vis'ualinstrumental aids'tonavigzlition.A In'apparatus of.' therst of these. classes, the control quantityis applied tocontrol'the operation of power apparatusto exert a' controliaction effectiveto alter the attitude',-speed, or path of'traVeLV-oflzth'e craft, and thereby realisev ani .automaticV control' system for the craft; In apparatus' of the second"y class the control quantity is-applied to controlfthe -di'splacement ofan index inan indicating instrument, which'V displacement thereby conveysvisual information toV the pilot or navigator of thefcraftgsenabling Ahimto 'control'thew attitude-speed,v or path of travel,` of the craft to' navigate' itin a desiredmannen Inleither case, theinformation representedbythecone trol quantity isin general-Linfo'rmation concerning a cha-r`` acteristicfo the-crafts position or motion: thus `it may be infomation concerning the'. crafts attitude; direction off'rnotion; or speed,` oreconcerning its: direction, distance,` or-fpositiom:vvithlfrespectto'some external point, object", on: surface. Alternatively,` the information:l represented fi byl thercontrol'. quantity-:maylfbe compounded out! ofil la A numbenfofitemsof informationrofA this kindfz thus; inil oneaknown'apparatns.i of. thiskind: for assisting inrthecona troli of .aircratc.the-l control! quantity' iss the: algebraic :surris-T ofnthreeuqnantitieg. one..proportional to: the` aircrat'tisldise` tance from a track to be followed:deline'd.`byf.externals craftgitofoperate:aecontrol surfaceaof the aircrafttobanlef: thegaircraftu. Sincefthe.; controlr-actionrisz usually= a1.-cor rectingaaction :tot correct a deviations ofrfthescraftr from: desired conditionizofrftravelgsinceathisfcorrectn .if actions mayf be; i initiated. in;'.either` sense *according vas ther: deviationrfi infonefsensefor .the otherfrmxthei..desired-1f conditionnof traVel;.e.-g:. deviationz-ot 'an aircraft: infonet.; sense tor-the fother, from. a :path than it is desiredihat thee` aircraft shouldrfollovsf;` and: snee-` no correcting: action is.' i required ifrv the .desired conditionzfis actually 1 obtaining, .itf is t-.usnal for` theifcontroli quantity. ytobel zero rWhensthefde-w sirel-.conditionaof flight obtains. and for -itl to..depart from. zero ine-onefsense` orthe-other according'. as thee--aircraft;.- departs infoneesense. and/ or thenother. from: thewzdesiredn condition-@of ightw The;-.invention is. specifically-const cerned with apparatus of thisakinds. Apparatuszoffthisrf: kimixincludewfinrparticulai.;:apparatussirriwhiclrstheicontrol quantity is used to deect an indicator that is nor- Y Patented: JunewQ; 1959 ICG 2f mally in .the centreV of 'its range of .movement wh'enith'e desired condition. vof flight is obtained, and which isdef. ii'ectedA to.one.sid'e orthe other .by the control-quantity., accordingasthe departure of .the aircraft fromthe desire L conditionisin. one sense or. the other. Suchindicators` are referredto. as centre-zero instruments. By analogy, apparatus of tthe kind'speciied maybereferredto as.. centre-zero controlv apparatus.-

A4 diiiiculty arises.- in the use ofall such'..centre,zero. control apparatus. This isthat, .iffthere is a .failurein the system of a kind that results in the control quantitys not being. transmitted through its appointed Vsignal chan-1 nel to appear Where it is ,to exert its `control action, theA indication or information provided, or control actionex.- ercised is that appropriatewhen the craftV is b'eingfcorrectly controlled to maintain the desired condition of! travel; In other Words, the system does not discriminate between complete accuracy. of control and certain kinds of complete failure. Y

For example, ina centre-zerov indication apparatus intended to indicate zero when the aircraft is following` a guide-path,` failure of the controll quantity to appear at the indicator may lead the` pilot to believe that he is accurately onthe guide-path when he is in fact a long,` wayfrom it. This may be highly dangerous.

Theliability of the apparatus to experience failures of. the kind vconsidered is obviously greater in apparatus in which. the signal transmission channel for the control quantity is long and elaborate. In some systems,.e.g. those inwhichlthc control quantity is used to exercise-.automatic control of the aircraft, but also in some in which`H the controlqna'ntity is applied solely to a visual-indication instrument,'th'e.channel may include an electronic ampli? iier of 'possibly considerable complexity, failure of any componentof which may. cause the channel'toy cease to transmit..

Iti'san objectof. the invention. to providemeans asso' ciated'withcentrelzero control apparatusto test the operation' of the system continuously'during 'operation of. 'the apparatusto provide an'indica'tion Whether' the signal transmissionA channellfor theY control' `quantity is operating correctly, orto provide an immediatewarning if the transmission channel should tail to transmit signals.

According-to `theinvention there visrprovided inasso. cia'tionwithcentreezero controltapparatus means for 'cone'. tinuously'injecting into the signal. transmission channel'in" Which `thecontrol-quantity 'is transmitted a predetermined".

auxiliary' test signal togetherwith the controlsignal, this'f testsignal being adapted to be transmittedalong witlrthe` controlifsignal through the signal transmission"channel,;

and; yetto-"be separableor distinguishable fromthe condY trolsigna'lrfter passing fthroughthev channel, in'combina" tion"withindicating or control aparatus 'adapted tore"- ceiVe-"thecombination of `control signal and V"test" signal anditoe produce therefrom twocontrolA or indication eifects one distinctively due to the control'vsignaland 'the'other distinctively` due tol the Vtest signal, the latter" adapted`v .to

ser-veras an indication of operativeness ofthe signal trans? mission channel, so that absence of the said etect may` serve as; ortoprovide, a Warning that the signal" transmission channel hasceasedlto operate.

Preferably the auxiliary test signal is injected yinto' the signal;"transmissionvv channel for the control quantity as near to the beginningl oflthat channel, i.e. as'near tor-the` originyof the'control quantity, as possible,'and/the indicator is connected to that channel as far along (the channel," i.e;1," asfneanvv to the Lutilisationt `device' as 'possibles In particular` where-the system-is of the kind in which'theA` a fmanneriditinguishble from I its responsetoethe rcontrolfg l quantity, so that it itself constitutes the test indicator.

A switch may be provided to switch the test signal on or olf as desired, so that it may be continuously applied only in selected circumstances e.g., in the case of an aircraft, during a landing approach to an airfield.

The invention may be put into effect in two general methods. In the first method of realisation the control signal and the test signal are discriminated from each other by signalaselecting circuit means, the control signal being applied in one channel and the test signal in another. In this method the test signal may either operate an indicator or control a warning device, which is actuated on failure of the signal. In the second method of realisation, the control signal and the test signal are applied together to an indicator the responses of which to the control signal and to the test signal are markedly different to visual observation, and are recognisably distinct and distinguishable even when both responses are present simultaneously. Discrimination between the two signals in this method of realisation is effected visually by the observer, this being possible on account, ultimately of differences in character between the two signals and of differences in the effects of the two signals on the indicator.

In navigational apparatus in which the invention may be applied the control signal may be a D.C. voltage whose magnitude and polarity corresponds at any instant in magnitude and polarity fto the value at that instant of the control quantity that it represents, or it may be an alternating voltage at a substantially constant carrier frequency, e.g. 400 cycles per second, whose amplitude, and whose polarity with respect to a reference wave at the carrier frequency vary, in yaccordance with the value of the control quantity, so as to constitute a suppressed-carrier modulated carrier wave that has zero amplitude when the control quantity is zero, and that, at other instants, represents the control quantity by the instantaneous amplitude and polarity of the modulation wave envelope.

It will be appreciated that, in operation, the control quantity 'will be continually changing and may therefore be considered as consisting of a number of components having different frequencies within a band of operational signal frequencies. In particular, the control signals used in aircraft navigational systems may be considered as having frequencies in a range extending from l cycle per second to $450 cycle per second.

Accordingly it is found convenient to use as a test signal, in the case of a system in which the control signal is a reversible-polarity D.C. voltage, a voltage of substantially constant frequency and amplitude, the frequency being of the order of to 1000 cycles per second. Similarly, in the case of a system in which the control signal is a modulated-carrier wave, the test signal may conveniently be a voltage that also has the form of a suppressed-carrier modulated carrier wave having the same carrier frequency as the control signal, but having a substantially constant modulation frequency and amplitude the modulation frequency being of the order of 5 to 1000 cycles per second.

Two embodiments of the invention will now be described with reference to the accompanying drawings in which Fig. 1 shows diagrammatically an embodiment of the invention as applied to a control system for a control surface of an aircraft.

Fig. lA shows an alternative circuit arrangement for one portion of the system of Fig. 1.

Fig. 2 shows an embodiment of the invention as applied to navigational apparatus for aircraft.

Figs. 3 and 4 are representations of an indicator used in the apparatus of Fig. 2 showing diagrammatically the indication produced when the apparatus is functioning correctly and incorrectly respectively.

In Fig. 1 apparatus is shown diagrammatically for signals from a control device 4 which may be a pick-off mounted on a gyroscope so that it gives indications of pitch of the craft. 'I'he control system shown is one in which the control signals are reversible-polarity D.C. voltages, and the control device 4 is represented as a variable potential divider connected across D.C. supply lines 1 and 2. A fixed potential divider 3 is lalso connected across these D.C. supply lines and its centre point is connected to earth so that the potential on the movable arm of potential divider 4 may be made either positive or negative with respect to earth. 'Ihe movable arm of potential divider 4 is connected to the live input terminal of a D.C. mixing amplifier 8 through a high resistance resistor 5. The mixing amplier 8 may conveniently be of the kind shown in United States patent application Serial No. 130,328, tiled November 30, 1949, now Patent No. 2,730,573, and assigned to the assignee of the present application, and may have a negative feedback connection including the resistor 6 over its first stage, or stages, to maintain the input impedance of the amplifier at a Very low value as explained in the above mentioned patent application Serial No. 130,328, now Patent No. 2,730,573. The second input terminal of amplifier 8 is connected to earth, as also is one of its output terminals. The output of the amplifier is arrangedv to control an electro-mechanical transducer 9 which may conveniently comprise the electro-magnetic clutch, hydraulic valve, and hydraulic motor shown in United States application Serial No. 104,862, filed July 15, 1949, now Patent No. 2,644,427, and also assigned to the assignee of the present application. Alternatively the electromechanical transducer 9 may be an electric motor capable of being differentially controlled. In either case the ytransducer is so arranged that it operates to rotate the shaft 10 at a speed dependent on the magnitude of the control signal supplied to the input terminals of the amplifier 8 and in a direction dependent on the polarity` l of that control signal. Shaft 10 is arranged to control the elevator 26 through a coupling arrangement 11 and also operates the movable arm of potential divider 12 connected across the supply lines 1 and 2. The potential on the movable arm of potential divider 12 is fed to Ithe input terminal of amplifier 8 through a high resistance resistor 13, the connection being such that when the elevator 26 has moved the amount corresponding to the position of the movable arm of potential divider 4 the voltages on the movable armls of these two potential dividers are equal and opposite, so that the total input signal to amplifier 8 is zero, and movement of the elevator 26 consequently ceases.

According to -the invention there is provided a signal generator 14, the output of which is connected across the input terminals of amplifier 8 through a capacitor 15 and a high-resistance resistor 16. The signal generator may be an oscillator or a pulse generator audits operating frequency may be of the order of 500 cycles per second, but satisfactory operation of the system can be obtained with any operating frequency between, say, 20 cycles and 1000 cycles per second. When `a pulse generatorV is used, the pulses must be arranged to have equal excursions in the positive and negative directions with respect to earth, and to have equal durations during these excursions so that their integrated value over any appreciable period of time is zero. The oscillations or pulses from signal generator 14 are passed through the amplifier 8 and appear at the output terminals together with the D.C. control signal. Their frequency is arranged to be sufficiently high to have no effect on the electromechanical transducer 9 because of its mechanical inertia. If the characteristics of thetransducer are such that it would respond to the operating frequency chosen for the signal generator 14, a capacitor may be connected across the input terminals of the transducer to prevent its responding to A.C. signals.

In the arrangement according to the invention besides Y "essonne Vbeing connected to` the transducer 9 throughlresisto'r '7,

'the output of amplifiers is also connected to a frequencyresponsive relay Vdevice 118 through a-capacitor 225. ln vthe arrangement shown .this frequencyfresponsive device is a dynamometer relay having the output --of `amplifier 8 connected between terminal Avandterminall C and `thus across the xed coils 20 of,` the relay. "5I-he output of Vsignal generator 14 is also connected to-fthe relayl device 18 through resistor 17. As shown, it `iscon- 4nestedbetween terminals B and C and; thus 'lacrossthe moving' coil 19 of `the dynamometer relay. 'A -relay of this Atype'isA operated when the fixed and Ymoving -coils are energised by two currents having. a predetermined frequency and phase relationship. The arrangement `shown -is-suclr that when the signals from signal generator 14 and also from the output of amplifier 8 are Apresent-together, the relay is operated 4and the contacts' 21 and 22 are opened. The contact 21's.con:nected to earth, and the other contact 22 is connected to terminal -D ofV relay device 18 and thence `to .one pole of a battery 23. The other pole of battery 23 is connected to `a lamp or similar warningdevice .24., the second terminal of which is connectedto earth..,thr ough a-switch or relay contact '27 which may be opened when ltheequipment is switched oif. A connection may `,also be .taken from 'the junction of one terminal ofthe -battery 2.3 and one terminal of the lamp .Z4-'tothe coupling device 11 which may include an electromagnetic clutch arranged so as to connect the shaft to vthe .elevator 26 so long as the relay contacts 22. are open, b ut to ldisconnect the shaft and the elevator when these contacts are closed, so that in the event of failure of the amplier'S, the elevator 26 may .'be operated by the .manual control system withoutthe operatothaving to .overcome the resistance ofthe transducer 9. Thus, inlthe system shown, so long as the .amplifier Sis .func- Ati'oning correctly, the contacts 21 and 22 of relay'device 18 are held open so that the lamp or Warning device 24 is inoperative, and the shaft 10 is coupled to the elevator 26. If ampliierS -fails,the contacts 22 of'lrelay device 18 are closed so that warning device 24 is operated and elevator 26 is disconnected from shaft 10 to facilitate manual control.

In place of the dynamometer relay shown in relay device 18 in Fig. l a rectifier arrangement may be used in conjunction with a normal type relay having a single operating coil. As shown in Fig. 1A terminals A and C are connected to the ends of the primary winding of a transformer 61 which has a centre-tapped secondary winding. The two outside ends of the secondary winding are connected to metal rectilers 63 and 64, the other terminals of which are connected to resistor-capacitor combinations 65, 67 and 66, 68. Between the centretapping of the secondary winding of transformer 61 and the centre point of resistor-capacitor combinations 65, 67, 66, 68 is connected the secondary Winding of a transformer 62, the primary winding of which is connected between terminals B and C. The coil of a relay 69 4is connected across the series combination of capacitors 67, 68, and its normally closed contacts are connected to terminals D and C. The output of oscillator 14 is connected between termin-als B and C and as a result of the rectifying action of metal rectiers 63 and 64 produces equal and opposite charges across the two capacitors 67 and 68 and consequently does not by itself operate relay 69. When the -test signal from amplifier 8 is present between terminal A and terminal C a voltage having a magnitude dependent on the magnitude of this test signal, and a polarity dependent on its phase sense relative to the output of the generator 14 appears across relay D. The arrangement is such that when the amplier 8 is functioning normally, the voltage across the coil of relay 69 is sutlicient to open the contacts.

In the embodiment of the invention shown in Figure 2, as applied to part of a navigation system for an aircraft,

ythe V/vertic'alpointer 51 of -a centre-zero indicator Av549 `is operated'inaccordanee Vvvitha control vquantity that fis compounded'frorn anunrberV of component control quantities provided by control devices 31, 32 and -33, in such-a way ythat the pointer is in its zero or central position when the aircraft is on a -required ight path -or when Vthe pilote-has taken the action necessary to return thel craft to its Vflight-path. The control devices comprise the-rollA'pick-oif of a gyro vertical indicated atfl31, a localiser-receiver'indicated at 32'and the pickfolf of an azimuth gyroscope Vindicated at -33. The pick-ols 31 and 33 are energised from -a source of 400 c./s. supply 36 and their outputs consist of voltages alternating at 400 -c./s.,=the 'amplitudes and phase senses yof which l'depend on theV magnitude and Isense of the control quanftities that lmeasure deviation of the aircraft from a predetermined bank angle and a predetermined azimuthal 'direction respectively. `The output of the localiser Areceiver -3-2-is a D-.C. voltage -Whose magnitude `and ypt alarityindieates'departure ofthe aircraft in magnitude and sense-fromfa predetermined flight path. This D.C. jvoltagefis fed-:to a'modulator34wliich isfalso connected -to the 400 c./s. supply 36 @so that the output of the `rnodulator'cornprises a voltage alternating at 400 c./s. per second having-a magnitude and-phase sense dependent on the magnitude and 'sense of departure of the craft from the ight path. .These three signalsare mixed in a mixing amplifier 37 Whichmay be of any known kind `sliitablefor receivingV three alternating voltages as inputs Vand providingas an output lan alternating voltage whose instantaneous value at 'anytime is proportional to the algebraic Ysum of thefinstantaneous values of the three inputs. The output `o-f'this mixing amplifier is fed toa phase-senseeensitive rectifier 38 which consequently lprovidesaDsC. voltage output dependent in magnitude and polarity onthe algebraic sum of `the various control quantities supplied to the mixing amplifierf37. The output ofphase-sense-sensitive rectifier 38 is fed to one of the operating coils 50 of a crossed-pointer centre-zero indicating instrument 49 in sucha way that the pointer 5.1 is in its central or zero position when the algebraic sum of the control quantities provided by the control devices 31, 32 and 33 is zero, indicating that the aircraft is following the desired path.

According to the invention, an additional input is provided to the modulator 34 from a signal generator 35. The generator may be an oscillator or a pulse generator having an operating frequency of approximately 10 c./s. per second. modulator 34 through capacitors 40 and 41 and resistor 39 in such a way that its output circuit will not affect the D.C. voltage provided by the localiser receiver 32. The signal supplied to the modulator by the signal generator 35 must not contain any D.C. component and for this reason the oscillations and pulses should be symmetrical about the gear axis. The output of modulator 34 will thus be a 400 c./s. voltage modulated not only by the D.C. output of the localiser 32 but also by the A.C. or pulse output of the signal generator 35. The l0 c./s. per second signal 'will be amplified in mixing amplifier 37 and will appear after demodulation at the output terminals of pbase-sense-sensitive rectifier 38. It is then supplied direct -to the operating coil 50 of the indicator 49 together with the control signal. Consequently pointer 51 will oscillate about the position to Which it is moved by the control signal. As a result of the mechanical damping and the inertia of pointer 51 the amplitude of these oscillations will be comparatively srnall so that a slight trembling motion is imparted to the pointer which gives it a blurred or fuzzy appearance to visual operation. As this effect is superimposed on any deflection that may be imparted to the pointer by the control signal, the pilot is able to read the dellections 0f the needle and to control the craft in response to such deflections in spite of the presence of the test Its output termia'ls are connected to thesignal, and as the test signal has no D.C. component, the position about which the pointer is oscillated is the same as that which it assumes when no test signal is used. l

When the pointer is in the zero position because the aircraft is on the desired course, it will have the appearance indicated in Fig. 3. On the other hand when the pointer is in the zero position because the control signals are not being received by the indicator, it will have the appearance indicated inA Fig. 4 and the pilot will know that he must not rely on'the indications produced by the system.

What is claimed is:

1. In combination with indicating apparatus for aircraft having an indicator adapted to be controlled in dependence upon a control signal representative of an error in a flight condition of the aircraft from a desired flight condition, said control signal comprising a suppressed-carrier modulated alternating voltage having D.C. components varying in accordance with said error and said indicator including a meter having a pointer displaceable over a scale in an amount depending upon the magnitude and polarity of the D.C. component of said control signal, a signal transmission channel for transmitting said control signal to said indicator, said signal transmission channel including receiver means for supplying the demodulation components of saidcontrol signal to said meter, means for supplying an auxiliary test signal to said transmission channel to be transmitted to said indicator simultaneously with said control signal through said :transmission channel, said test signal comprising an alternating voltage having a frequency and amplitude so correlated to the mass of said` pointer as to produce a visible oscillation of said pointer in response thereto, whereby to produce a quiver of said pointer for all displacements thereof produced by the D C. components yof said control signal.

2. An aircraft instrument system including an lindicator having a pointer dsplaceable from a reference position for providing to the pilot a visual indication of a meas- ,ured quantity, means for producing a Acontrol signal varying in accordance with changes in Vsaid quantity, a 4signal transmission channel connected .to receive said control signal and lto displace said indicator from said reference, position in accordance therewith, a sourcey of periodically varying test signal of a frequency and a magnitude sufcient to producen small but readily discernible oscillation of the pointer ol? said indicator independent from the movement of said pointer in response to said control signal, said test signal sourcebeing so connected with said signal transmission channel that the control signal and the test signal both pass through said channel to produce a discernible oscillation of said pointer in response to said test signal and a displacement thereof in response to said control signal whereby simultaneously to supply both an indication of the measured quantity and an indication as to `the operativeness or lack of operativeness of said system.

References Cited in the le of this patent UNITED STATES PATENTS 1,087,189 Schweitzer Feb. 17, 1914 1,522,581 Espenschied Ian. 13, 1925 1,573,801 Bown Feb. 23, 11926 1,723,220 Ihorp Aug. 6,1929 y1,872,257 Durkee Aug. 16,1932 2,039,765 Beetem May 5, 1936 24,084,995 Barbulesco June 29, 1937 2,137,349 Rezos Nov. 22, '1938 2,164,161 Nekolny June 27, 1939 2,244,725 Peterson June 10, 1941 2,340,590 Hennig Feb. 1, 1944 2,421,081 OBrien May 27, 1947 2,477,028 Wilkie July 26, 1949 2,478,734 Abraham Aug. 9, 1949 2,685,684 Atkinson et al Aug. 3, 1954 FOREIGN PATENTS l 574,340 Great Britain Jan. 1, 1946 

